Starch Gelatinization

Starch gelatinization is a process that breaks down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. This irreversibly dissolves the starch granule. Penetration of water increases randomness in the general starch granule structure and decreases the number and size of crystalline regions. Crystalline regions do not allow water entry. Heat causes such regions to become diffuse, so that the chains begin to separate into an amorphous form. Under the microscope in polarized light starch loses its birefringence and its extinction cross. This process is used in cooking to make roux sauce. The gelatinization temperature of starch depends upon plant type and the amount of water present, pH, types and concentration of salt, sugar, fat and protein in the recipe, as well as derivatisation technology used. Some types of unmodified native starches start swelling at 55 °C, other types at 85 °C. The gelatinization temperature depends on the degree of cross-linking of the amylopectin, and can be modified by genetic manipulation of starch synthase genes.

During gelatinization, water acts as a plasticizer. Water is first absorbed in the amorphous space of starch, which leads to a swelling phenomenon during heating and then transmitted through connecting molecules to crystalline regions. Water enters tightly bound amorphous regions of double helical structures to swell amylopectin, thus causing crystalline structures to melt and break free. Stress caused by this swelling phenomenon eventually interrupts structure organization and allows for leaching of amylose molecules to surrounding water.

Gelatinization temperature also depends on the amount of damaged starch granules. These will swell faster. Damaged starch can be produced, for example, during the wheat milling process, or when drying the starch cake in the starch plant.

There is an inverse correlation between gelatinization temperature and glycemic index.

During gelatinization of starch, three main processes happen to the starch granule: granule swelling, crystal or double helical melting, and amylose leaching.

Gelatinization improves the availability of starch for amylase hydrolysis.